Nanotechnology switches back to vacuum transistors at low voltage

July 03, 2012 // Peter Clarke

Researchers at the University of Pittsburgh have come up with a device structure that allows a switch back to vacuum, in contrast to the solid-state, as the medium for electron transport in transistors.

The team is proposing a MOS vertical structure with a triple layer of metal/silicon dioxide/silicon exposed on the side by a deep trench. The metal and silicon layers form the anode and cathode of the device, separated by the insulating silicon dioxide, and the electron transport occurs in the vertical direction through the vacuum.

The work is discussed in a research paper entitled Metal-oxide-semiconductor field effect transistor with a vacuum channel, published in Nature Nanotechnology July 1.

The work represents a return to the roots of electronics. The solid-state transistor was invented in 1947 as a replacement for the bulky, unreliable vacuum tube. Vacuum tube style electronics in miniature made using solid-state semiconductor manufacturing techniques have been tried before, but the concept has struggled to overcome requirements for high voltage and issue of compatibility with the incumbent solid-state CMOS technology.

A team under Hong Koo Kim, principal investigator on the project and a Professor in the University of Pittsburgh's Swanson School of Engineering, has redesigned the structure of the vacuum electronic device. With the assistance of PhD candidate Siwapon Srisonphan and postdoctoral fellow Yun Suk Jung Kim and his team discovered that electrons trapped inside a semiconductor at the interface with an oxide or metal layer can be easily extracted out into the air. The electrons at the material interface form a sheet of charges, a two-dimensional electron gas and Kim found that the Coulombic repulsion of the electrons for each other enables the easy emission of electrons out of the silicon.

This allows the creation of a low-voltage device in which the electrons travel ballistically in air in a nanometer-scale channel without any collisions or scattering.

The channel length is of the order of 20-nm and the team measured a transconductance of 20-nS per micron and an on/off ratio of 500 and turn-on gate voltage of 0.5-V under ambient conditions, according to the paper's abstract.

"The emission of this electron system into vacuum channels could enable a new class of low-power, high-speed transistors, and it's also compatible with current silicon electronics, complementing those electronics by adding new functions that are faster and more energy efficient due to the low voltage," said Professor Kim, in a statement.

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